The invention pertains to the integration of semiconductor x-ray radiation sensors within a Scanning Electron Microscope (SEM) or similar analytical electron-beam instrument, optionally in conjunction with a backscattered electron sensor. The invention pertains to novel methods of configuring both the detection elements and the microscope so as to achieve improvements in performance and economies of construction, as well as other benefits.
The installation of a solid-state Energy Dispersive X-ray (EDX) detector onto an electron microscope was first reported by Fitzgerald, Keil, and Heinrich in 1968. The type of detector described was a lithium-drifted silicon (Si(Li)) diode that was introduced through the port of an electron probe micro analyzer (EPMA). This kind of detector was soon commercialized and units of this same general type have been installed on many kinds of Electron Microscope (EM), notably including the Electron Probe Micro-Analyzer (EPMA), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and Scanning Transmission Electron Microscope (STEM). Though the technology has been greatly refined over the years, the EDX units themselves have retained certain significant characteristics of the earliest models.
FIG. 1 is a schematic representation of a conventional Si(Li) EDX detector unit [60]. The active sensor element [1] is the Si(Li) diode and installed in close proximity is the sensor's field effect transistor (FET) [2] which amplifies the current pulse produced by the sensor element. Both the sensor element and the FET are mounted on the anterior end of a thermally conductive rod [3] that is in thermal contact at its posterior end with a reservoir of liquid nitrogen [4] contained in an insulated container called a dewar. The thermally conductive rod (“cold finger”), the sensor diode, and the FET are all enclosed in a tubular hermetic envelope [5] that is maintained at a high vacuum, both to minimize thermal transfer, and to maintain the sensor in a clean condition. The anterior end of the tubular hermetic envelope is closed with a thin window [6] that admits x-rays to the front face of the sensor but maintains the interior vacuum. The liquid nitrogen (LN) reservoir [4] is an essential aspect of the design of the conventional Si(Li) detector since it is required that the sensor element and FET be maintained at a cryogenic temperature if acceptable energy resolution is to be achieved. A collection of circuitry conventionally known as a “preamplifier” [61] receives and further amplifies the signal received from the sensor FET [2] as well as providing additional electronic functions in support of the sensor operation. In order to implement a complete EDX system, the detector unit of FIG. 1 would be supplemented with additional electronic components. Such components conventionally provide for necessary electrical power, digitization of the waveform received from the preamplifier [61], collection and display of the resulting x-ray spectrum, and spectrum analysis functions. Such additional support components are known in the art and are not the focus of the present invention.
FIG. 2 schematically illustrates the manner in which a conventional Si(Li) EDX detector unit [60] is installed on a conventional SEM or EPMA (which are physically similar instruments). The EDX detector unit is mounted on a flange [7] that is bolted onto one of the several ports usually provided for such use on the specimen chamber [8] of the electron microscope. The sensor end of the EDX detector [60] is thus brought into close proximity to the specimen [9] (positioned on specimen stage [9′]), to intercept the x-rays emitted at the point of impact of the focused electron beam [10]. The electron beam is produced by the electron optics “column” [11]. A backscattered electron (BSE) detector [62] is commonly mounted beneath the final focusing lens [11′] of the electron column [11]. The specimen chamber [8] is shown with one side open for purposes of this illustration, but in use, the specimen chamber must be sealed and evacuated (and is thus interchangeably known as the “vacuum chamber”). Thus, the detector tube [5] of the detector unit [60] must be vacuum-sealed at the flange [7] where it passes through the vacuum chamber wall. Customarily this involves a radial o-ring seal to the exterior of the tube, and typically this is designed as a sliding seal so that the insertion of the detector relative to the specimen may be varied. Thus the cylindrical tube [5], the flange [7], and the “cold finger” [3] function in a complementary fashion to support the sensor element [1] in its desired relation to the specimen. This support system may be configured in a variety of ways, but it is physically anchored by the flange [7] which is mounted to the exterior of the vacuum chamber [8].
The above figures and explanations describe, in a very general manner, the standard design of the Si(Li) EDX detector and the manner in which it is installed on an electron microscope of the SEM/EPMA type wherein the specimen stage [9′] carrying a bulk specimen [9] is located beneath the final probe-forming lens [11′] which constitutes the bottom element of the electron optics column [11]. Si(Li) detectors of similar design have also been employed with electron microscopes of the STEM/TEM type wherein a small electron-transmissive specimen is supported in the gap of an objective lens. Though certain specialized considerations may pertain to EDX detectors employed for STEM/TEM types of instrument, those detectors too have also generally conformed in the historical practice to the principles of construction illustrated in FIG. 1.
Pertaining to the interface of an EDX detector to an electron microscope of the SEM/EPMA type (which is the field of the subject invention), there are several important considerations affecting the performance of the detector:                (a) A large detection solid angle is desirable in order to maximize the number of detected x-rays (and thus the statistical precision) that can be achieved for a given beam current and measurement time.        (b) A high take-off angle is desirable in order to minimize absorption effects as the excited x-rays exit from a point of origin below the surface of the specimen.        (c) Optimal collimation of the detected x-rays is facilitated by pointing the detector coaxially at the intended beam impact point on the specimen. Such collimation ensures that only the uniformly responsive area of the sensor is employed for detection and that x-rays originating from scattered electrons are excluded.        
Thus, in an “ideal” situation, the sensor element would be located very close to the specimen, with its axis in line with the intended beam impact point, and inclined at a high take-off angle. However, it is also desirable for the focusing lens of the microscope to be in close proximity to the specimen and, for the many applications in which a backscattered electron detector is required, that the BSE detector's view of the specimen should not be obscured. Thus, the space under the focusing lens is both small and crowded and this restricts the attainment of ideal detector geometry. Further, the physical arrangement of the specimen chamber, such as the presence of access doors and auxiliary ports, will play a large role in restricting where and how detectors may be placed.
It can be appreciated that the above considerations, when coupled with the over-riding necessity of thermally coupling the Si(Li) sensor to an external cryogenic cooler, have shaped the evolution of the traditional EDX detector unit into the familiar tube-mounted configuration illustrated in FIG. 1 (which has sometimes been descriptively referred to as a “sensor on a stick”). In turn, the standardization on this kind of tube-mounted configuration has also affected the design of electron microscopes which, by necessity, are specifically configured for mounting of such detectors. Since x-ray detectors have historically been designed and manufactured by one group of suppliers and electron microscopes by another, departure from this familiar model has not been attractive to either group.
Within the past decade, the technology of EDX detectors has been radically altered by the introduction of highly capable x-ray sensors that do not require cryogenic cooling. The principal current embodiment of this type of detector is the so-called “silicon drift detector” (SDD) whose operation is described in the scientific literature. These devices achieve spectroscopic performance generally superior to that of the Si(Li) detector, but at temperatures that can be conveniently achieved with a small thermoelectric cooler (TEC) based on the Peltier principle.
FIG. 3 illustrates the basic structure of a packaged SDD module as may currently be purchased from a manufacturer (PNDetector GmbH of Munich, Germany) of such devices. Within such a module, the SDD sensor element [12] is a small planar “chip” manufactured by semiconductor processes. The SDD sensor chip is mounted on the cold face of a small TEC device [13] which is attached to a housing base [14]. A thermally conductive stud [15] is in thermal contact with the warm side of the TEC device and serves as the external “sink” attachment by which heat generated by the TEC is removed from the module. A collimator plate [16] is typically mounted in front of the SDD sensor element [12] so as to permit x-rays to strike only the intended active area. A housing cap [17] is sealed to the housing base [14]. The front of the housing cap [17] is closed with a thin window [18] which permits x-rays to enter and strike the SDD sensor element [12]. The housing base [14] is provided with an array of electrical connection pins [19] through which power and control signals may be provided to the sensor and the TEC, and through which output signals may be extracted (connections of these pins to the internal elements are omitted from the figure). One of the signals available via the output pins is the front-face temperature of the TEC module [13], which permits external circuitry to regulate a constant operating temperature for the sensor element [12] (the temperature sensor required for this regulation is not shown in the figure). The housing base [14] is typically configured according to the “TO-X” convention that has been used within the electronics industry for mounting various types of devices and sensors (e.g., the TO-8 case). The entire module is hermetically sealed and may be evacuated or filled with an inert gas at a reduced pressure.
The FET which amplifies the weak current pulse of the sensor remains a key component of the SDD device and must be located in close proximity to its anode electrode, just as for the Si(Li) technology. However, rather than implementing the FET [2] as a discrete element as illustrated in FIG. 1, one manufacturer of SDD sensors integrates the FET circuitry into the same semiconductor die [12] as the sensor itself, and this is the type of device illustrated in FIG. 3. Other manufacturers mount a discrete FET in close proximity to the anode. Regardless of whether the FET is integrated or discrete, however, it is understood that a FET is associated with the SDD sensor.
The use of the kind of packaged SDD module here illustrated is not required for the implementation of an x-ray detector based on SDD technology. Indeed, it is believed that at least one manufacturer of EDX detector units places the unpackaged elements of the sensor module directly on the end of a cold finger and encloses it in an evacuated tube, thus closely mirroring the conventional construction of the Si(Li) detector illustrated in FIG. 1. There is both specialized art and specialized equipment involved in mounting and packaging the SDD sensor and, to date, the majority of x-ray detector manufacturers have chosen to purchase pre-packaged modules from one of the specialized suppliers of such modules. Thus, for the remainder of this discussion the use of a packaged SDD module, such as illustrated in FIG. 3 will be assumed. However, it will be apparent that the same principles taught in the subject invention could be equally applied to the construction of a detector assembled from unpackaged components.
In order for the packaged SDD module of FIG. 3 to be made into a functional x-ray detector unit, it must be provided with several things:                1) Power supplies and control signals to operate the SDD sensor and its incorporated FET.        2) Preamplifier circuitry to amplify the weak signal from the sensor FET so as to produce a robust waveform that can be quantified.        3) Temperature control electronics that actively regulates the power provided to the internal TEC element [13] so as to maintain the desired operating temperature of the SDD sensor element [12].        4) Mechanical elements to support the module in the required proximity to the specimen.        5) A thermal path to a thermal sink of sufficiently low temperature to which the thermal stud [15] may discharge the heat generated by the module.        
To date, commercial SDD detectors have accomplished these provisions in a package that closely emulates the format of the traditional Si(Li) detector. FIG. 4 depicts schematically a conventional SDD detector employing a packaged SDD module. The packaged sensor module [20] is attached to the anterior end of a suitable thermally conductive rod or pipe [21], whose posterior end terminates with a heat dissipating mechanism that is incorporated in the body of the detector which is exterior to the vacuum chamber. A housing tube [71] encloses the thermally conductive rod [21] and provides the sealing surface by which the detector snout maintains a vacuum-tight connection as it passes through the mounting flange attached to a microscope (e.g., flange [7] of FIG. 2). A thermal insulating element [72] provides thermal isolation of the housing tube [71] from the thermally conductive rod [21] while providing mechanical support. The heat dissipating mechanism incorporated in the body of the detector exterior to the vacuum chamber might take various forms (for example, in one known case, a chilled water cooler has been employed). In the specific case illustrated, the thermally conductive rod [21] is terminated with a plate [73] which abuts the “cold” side of a second TEC element [22]. The “warm” side of the second TEC element [22] is in contact with an air-cooled heat sink [23] by which the heat generated by the second TEC element is dissipated to the environment. This heat sink, equipped with fins or similar structures to facilitate convection, is typically integrated into the exterior case of the detector unit. Contained in this case is a collection of electronic circuitry elements [24] which operate the detector and the second TEC element and provide the “preamplifier” function. The above is, of course, a highly generalized illustration of the internal components of a complete SDD detector and is subject to variation in detail, but nonetheless generally conforms to precedents established by the conventional Si(Li) detector:                Firstly, it is apparent that this conventional design adheres to the precedent of the LN-cooled Si(Li) detector, in that the sensor is brought into proximity with the specimen by inserting it into the specimen chamber at the end of a straight tubular element whose form and function is equivalent to that employed by the conventional Si(Li) detector.        Secondly, the thermal circuit of the conventional SDD unit continues to utilize the same “cold finger” strategy employed by the thermal circuit of the Si(Li) detector. Though its role in the SDD is to act as the heat-extraction conduit required by the embedded TEC cooler [13], rather than to directly cool the sensor, as for the Si(Li), in both cases the cold finger provides the thermal connection to an external heat-dissipating sink which is isolated from the specimen chamber.        Finally, like the Si(Li) detector, the conventional SDD detector is a modular unit which positions the sensor by means of a support system, including a tubular enclosure, which is again anchored externally by a flange attached to the vacuum chamber of an electron microscope.        
In short, in transitioning from Si(Li) sensor technology to SDD sensor technology, detector manufacturers have effectively retained the conventional detector design with the substitution of: (a) an SDD module for the Si(Li) diode and (b) typically a TEC module and air-cooled heat sink substituted for the LN dewar. This has been a rather logical migration path for both EM manufacturers and EDX detector manufacturers since it retains the conventional format of the Si(Li) detector and this has a number of commercial benefits in terms of compatibility with past and present microscope designs. However, this conventional practice does not take advantage of the opportunities to effect improvements in performance and cost reduction that are created by the altered constraints associated with an x-ray detector that does not require cryogenic cooling.